1. Field of the Invention
This disclosure relates to optical communication networks, and in particular to networks having self-amplified optical communication systems and methods.
2. Description of the Related Art
Optical amplifiers are used in optical communication systems for providing improved bandwidth-distance product of the system. An optical amplifier may be in the form of an amplifying or gain region(s) which is optically pumped by a dual-use light source that also provides signal light to the optical communication system. Such an optical communication system is herein defined to be a self-amplified optical system. A communication network incorporating such self-amplification is herein defined as a self-amplified network, abbreviated "SANE". If pump light passes through the amplifying region in the same direction as the signal light, the amplifier is said to be co-pumped, while if pump light and signal light travel in opposite directions, it is counter-pumped. If the pump light is provided from both directions, it is bidirectionally-pumped.
Freedom to locate amplifying regions anywhere within the network is useful in optimizing signal-to-noise ratio, signal linearity, and robustness of the network. However, the locations for optical sources may have to be fixed because they are electrically powered and need to be accessible for maintenance. Remote optical pumping of amplifiers provides advantages for such operating concerns. See, for example, P. B. Hansen et al., "423-km Repeaterless Transmission at 2.488 Gb/s Using Remotely Pumped Post- and Pre-Amplifiers", EUROPEAN CONF. ON OPTICAL COMM, Florence, Italy, Sept. 1994. Remotely pumped optical amplifiers can also be deployed at critical locations in networks which are passive; i.e. networks which require no electrical power except at the terminal locations.
Gain reduction of the optically pumped amplifiers is a concern if the pump light intensity falls too low for a time period longer than the gain relaxation time of the amplifier. To avoid this, a modulation format for each dual-use signal/pump source may be chosen which avoids long periods of low intensity. Alternatively, network multiplexing protocols may be chosen to ensure that periods of low gain for each amplifier are restricted to times when it is not amplifying signals.
Amplifiers based on optical non-linearity can operate over a wide range of wavelengths. Typically, conventional silica fibers constitute the active medium, although bulk materials are also possible. Amplifiers based on fiber non-linearity have a strong dependence on pump power, which must be fairly large for useful amplification to occur. Brillouin scattering has the lowest threshold for pump power and is insensitive to polarization. For example, R. W. Tkach et al., "Optical Demodulation and Amplification of FSK Signals Using AlGaAs Lasers," ELECTRONICS LETT., VOL. 24, 1988, pp. 260-262 discloses a system having optical amplification and wavelength channel selection in point-to-point links. Signal and pump wavelengths are closely spaced, which makes tunable sources practical. Raman scattering and other fiber non-linearities can also serve as amplification mechanisms. Raman scattering is discussed, for example, in J.-P. Blondel et al., "Theoretical Evaluation and Record Experimental Demonstration of Budget Improvement with Remotely Pumped Erbium-Doped Fiber Amplification", IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 5, NO. 12, Dec. 1993, pp. 1430-1433.
Amplifiers based on rare-earth-doped fibers may be operated at specific pump and signal wavelengths in specific bands to take advantage of special material characteristics. For example, fibers doped with Erbium (Er) amplify signals at wavelengths near 1550 nm with good noise figure, when they are pumped by lasers operating at 980 or 1480 nm. They have a long excited-state lifetime, usually greater than 1 msec.
U.S. Pat. No. 5,299,048 to Suyama discloses a self-amplified optical system. A light source acts as a fixed wavelength pump and also as a signal source in point-to-point links and the use of amplifiers based on rare-earth-doped fibers is suggested. These amplifiers are locally-pumped and the dual-use pump/signal light sources are modulated by a subcarrier method, with subcarrier periods much less than the fluorescence time of the rare-earth-doped fiber.